Thin film optoelectronic devices have experienced significant advancement over the last decade. Light emitting diodes (LEDs) based on organics, polymers, and quantum dots have achieved high efficiencies and long lifetimes suitable for applications in full color displays and solid-state lighting. Organic/polymeric photovoltaic cells (PVs) have been established as a promising low-cost solar energy conversion technology with power conversion efficiencies improved from about 1% to more than 10%. Typically, optoelectronic devices are configured with a layered structure, with the photoactive (either light emitting or light harvesting) layer sandwiched between charge transport layers in contact with two electrodes. The charge transport layers, i.e. hole transport layer (HTL) and electron transport layer (ETL), may play important roles in determining device performance.
A desirable hole transport material can have suitable energy levels, with large band gap and high hole conductivity, efficient hole injection and transport, as well as electron and exciton blocking. A variety of solution processable hole transport materials have been developed during the last decade, including organic molecules, metal oxides, and polymers, e.g., poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Of these, PEDOT:PSS might be the most recognized material due to its good conductivity, high transparency, and suitable work function.
However, there are many intrinsic limitations associated with PEDOT:PSS. For example, its acidity and/or hygroscopic nature can lead to device instability and/or degradation, and its low lowest unoccupied molecular orbital (LUMO) energy level and band gap can result in weak electron blocking and/or pronounced exciton quenching. Crosslinkable organic/polymeric hole transport materials, which allow for the formation of a solvent-resistant layer via crosslinking after solution processing, have been explored for multilayer structured devices, in particular OLEDs. However, the preparation of crosslinkable materials often is not straightforward, and typically involves multiple synthesis and/or purification steps, which can be costly. Transition metal oxides, such as oxides of nickel (NiOx), molybdenum (MoOx), tungsten (WOx), and vanadium (VOx), represent another class of hole transport materials that has been pursued. Solution processed metal oxide HTLs are mainly obtained by either thermal decomposition of organic-inorganic hybrid precursors or annealing of nanoparticles capped with organic solubilizing/stabilizing groups. Important issues related to solution processed oxide thin films include defect states due to stoichiometry deviations, residual —OH groups, and/or organic residues, which can negatively impact the device efficiency and stability.
Recently, earth-abundant organometal halide perovskites have attracted attention because one or more of their properties, such as optical and/or electrical properties, can make them suitable for low-cost high-performance optoelectronic devices. These properties can include facile low-temperature synthesis, solution processability, highly tunable direct band gaps across the visible to infrared regions, and/or extremely high charge carrier mobilities. Success has been realized for perovskite based PVs, with device efficiencies increasing from about 3% to about 20%, or more. Electrically driven LEDs and optically pumped lasers also have been demonstrated with these organic-inorganic hybrid semiconductors.
Methylammonium (MA) lead chloride (“CH3NH3PbCl3” or “MAPbCl3”) has optical and electronic properties suitable for application in hole transport layers, i.e. transparency in the visible region due to a wide band gap of about 3.1 eV, high conductivity, and high hole mobility. Methylammonium lead chloride also can be synthesized by reacting MACl with PbCl2, in the same way as other methylammonium lead halide perovskites, such as MAPbI3 and MAPbBr3.
However, preparing high quality neat MAPbCl3 thin films via solution processing is more challenging than preparing high quality neat films of MAPbI3 or MAPbBr3. Not wishing to be bound by any particular theory, it is believed that this difference may be due to the lower solubility of chloride precursors and/or faster crystallization kinetics. Due to one or both of these features, spin coating a dimethylformamide (DMF) precursor solution containing MACl and PbCl2 typically leads to the formation of relatively large MAPbCl3 crystals with poor surface coverage and/or roughness.
Therefore, films of, or containing, methylammonium lead chloride crystals that do not suffer from one or more of the foregoing disadvantages are desired.